The kidney contains several distinct epithelia that, in their aggregate function, are responsible for formation of the urine and the maintenance of fluid and electrolyte homeostasis. We are studying the roles of these epithelia in the regulation of the excretion of water, urea, ammonium, bicarbonate, sodium, potassium, and chloride. The general approach is to dissect the epithelia from the kidney and to study their functions in vitro. These studies measure the transport function of the epithelia, the activities of various enzymes which catalyze critical chemical reactions, and the levels of nucleic acids (mRNA) that are involved in the regulation of both the transport and metabolic functions of the epithelia. The data are analyzed and integrated using mathematical models of transport in the kidney.